Such a process is known from EP-A-776959. This publication describes a process wherein the high boiling fraction of a Fischer-Tropsch synthesis product is first hydroisomerised in the presence of a silica/alumina supported Pd/Pt catalyst. The isomerised product having a content of non-cyclic iso-paraffins of more than 80 wt % is subsequently subjected to a pour point reducing step. The disclosed pour point reducing step in one of the examples is a catalytic dewaxing step performed in the presence of a silica supported dealuminated ZSM-23 catalyst at 310° C.
A disadvantage of such a process is that only one grade of base oils is prepared. A next disadvantage is that the hydrosiomerisation step is performed on a narrow boiling range fraction of a Fischer-Tropsch synthesis product, which hydroisomerisation step is especially directed to prepare a base oil precursor fraction having the desired properties. The hydroisomerisation process step can also yield valuable middle distillates next to base oil precursor fractions if the feed would also include more lower boiling compounds. There is thus a desire to prepare base oils from a waxy paraffinic fraction as obtainable from a hydroisomerisation process step which yields both middle distillates, such as naphtha, kerosine and gas oil, and the waxy paraffinic fraction having a content of non-cyclic iso-paraffins of more than 90 wt %. There is also a desire to have a flexible process wherein two or more base oils having different viscosity properties are obtained of excellent quality.
WO-A-0014184 discloses a process to simultaneously prepare a gas oil and a base oil from a Fischer-Tropsch wax by hydro-isomerisation followed by catalytic dewaxing. The examples illustrate how a Fischer-Tropsch wax having almost no molecules having more than 60 carbon atoms is used as feed. A feature of this process is that the cold flow properties of the gas oil fraction could be improved.
U.S. Pat. No. 6,165,949 discloses a process wherein starting from the same FischerTropsch feed as illustrated in the above WO-A-0014184 a relatively low viscosity base oil is prepared by hydro-isomerisation and catalytic dewaxing. The base oil having a kinematic viscosity at 100° C. of 4.83 cSt is used in a wear resistant lubricant formulation.